In many processes, for instance, in the field of industrial processing engineering, chemistry or machine construction, fluid media, especially masses of gas (e.g. an air mass) having particular properties (such as temperature, pressure, flow speed, mass flow, etc.) have to be supplied in a defined manner. Among these are, in particular, combustion processes, which run under regulated conditions.
An important application example is the combustion of fuel in internal combustion engines of motor vehicles, especially ones having subsequent catalytic exhaust gas purification, in which a certain air mass has to be supplied per unit time in a controlled manner (air mass flow). Various types of sensors are used to measure the air-mass throughput. One conventional sensor type is what is generally known as a hot-film air mass sensor (HFM), which is described in one specific embodiment in German Patent Application No. DE 196 01 791 A1, for example. A sensor chip, which has a thin sensor diaphragm, e.g., a silicon sensor chip, is generally utilized in such hot-film air mass meters. At least one thermal resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors), is typically situated on the sensor diaphragm. In an air flow, that is guided over the diaphragm, the temperature distribution changes, which in turn is detectable by the temperature measuring resistors and is able to be analyzed with the aid of a control and evaluation circuit. Thus, for instance, an air mass flow is able to be determined from a difference in resistance of the temperature measuring resistors. Various other variations of this sensor type are available.
One problem with such a type of conventional sensor described in German Patent Application No. DE 101 11 840 C2, for instance, is that contamination of the sensor element may often occur, such as contamination by water, oil or other fluids, or other types of soiling. As a rule, the sensor chip is used directly in the intake tract of the internal combustion engine or in a bypass to the intake tract of the internal combustion engine. During operation of the internal combustion engine, in this context, water or oil may deposit on the sensor chip, and in this instance, on the sensor diaphragm, in particular. This soiling deposit can lead to an undesired effect on the measuring signal of the sensor, especially since a fluid film on the surface of the sensor affects the thermal conductivity of the surface, which results in a falsification of the measuring signals. Besides in hot film air mass meters, similar problems also occur in other types of sensors, which are used for measuring fluid parameters in a fluid flow.
In order to solve these problems with respect to contamination, and especially to prevent water and oil from reaching the sensor chip, various attempts are known from the related art. One attempt, which is described, for instance, in German Patent Application Nos. DE 101 35 142 A1 or DE 10 2004 022 271 A1, is the use of a so-called “bypass”. In this connection, a plug-in sensor is plugged, using a plug part, into an intake pipe, an intake opening being provided at the inflow side in the plug part. A main flow channel is provided in the plug-in sensor, through which a substantial part of the flow flows from the intake opening to a discharge opening. At a branch point, which in German Patent Application No. DE 10 2004 022 271 A1 is additionally developed as a sharp edge (designated also as a “nose”), a bypass channel, in which a sensor chip is situated, branches off from the main channel. The bypass channel eventually opens out into an outlet opening at the lower side of the plug-in sensor. The sharp edge at the branching point of the bypass channel has the effect, in this instance, that water droplets, because of their mass inertia, are able to penetrate into the bypass channel only with difficulty, and that they remain predominantly in the main channel.
However, in the conventional plug-in sensor constructions, it is disadvantageous that the plug-in sensors, because of their aerodynamically unfavorable shape, in many cases cause problems in the intake tract, with regard to a flow resistance-conditioned loss in pressure. In addition, the signal reproducibility of such sensors is comparatively low. For this reason, German Patent Application No. DE 10 2004 022 271 A1 proposes a construction in which a flow diversion part is provided as a separate component, in a fixedly installed manner in the flow pipe. As an alternative, a design of the flow diversion part in one piece with the plug-in sensor is also proposed. Furthermore, a flow-conducting wall is fixedly installed in the flow pipe, which is intended to calm the flow after the plug-in sensor. The construction described in German Patent Application No. DE 10 2004 022 271 A1 is, however, connected with various disadvantages, in practice. One disadvantage is, for instance, that the flow-diversion part is, generally, fixedly installed in a section of the flow pipe. This fixed installation brings on additional costs in the manufacture of these pipe sections, just as does the provision of the additional flow-conducting wall. In the development as one piece, the installation length is also too long for many applications. In addition, the construction described in German Patent Application No. DE 10 2004 022 271 A1 may be further optimized with respect to the pressure drop at the plug-in sensor.
Further problems are that the construction described in German Patent Application Nos. DE 101 35 142 A1 or DE 10 2004 022 271 A1 has only a comparatively low air mass throughput through the bypass channel, so that overall the signal level swings (and, with that, also the signal to noise ratio) are often comparatively unsatisfactory. In addition, in German Patent Application No. DE 101 35 142 A1 and DE 10 2004 022 271 A1 a construction is used in which the discharge opening of the main flow channel is positioned laterally on the plug-in sensor in such a way that outflowing air acts negatively on the course of the flow about the plug-in sensor, and thus additionally increases the flow resistance of the plug-in sensor or is even able to influence the signal characteristic of the plug-in sensor.